In this article we will discuss about:- 1. Meaning of Glass 2. Structure of Glass 3. Constituents and Functions 4. Properties 5. Furnaces 6. Fabrication 7. Classification 8. Special Types 9. Uses of Glass.
Contents:
- Meaning of Glass
- Structure of Glass
- Constituents of Glass and their Functions
- Properties of Glass
- Glass Furnaces
- Fabrication of Glass
- Classification of Glass
- Special Types of Glasses
- Uses of Glass
1. Meaning of Glass:
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Glass is any substance or mixture of substances that has solidified from the liquid state without crystallization. Elements, compounds and mixture of wide varying composition can exist in the glass state, but the term “glass” as ordinarily used refers to material which is made by the fusion of mixture of silica, basic oxides and a few other compounds that react either with silica or with the basic oxides.
No definite chemical compounds can be identified in glass. Many of its properties correspond to those of a super-cooled liquid whose ingredients cannot be identified because they have not separated from the solution in crystalline form.
Glass may also be defined as a hard, brittle, transparent or translucent material chiefly compound of silica, combined with varying proportions of oxides of sodium, potassium., calcium, magnesia, iron and other minerals.
Glass is an amorphous substance having a homogeneous texture.
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2. Structure of Glass:
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The glass is a random arrangement of molecules, the great majority of which are oxygen ions bounded together with the network forming ions of silicon, boron or phosphorus. A glass made of silica alone has many desirable characteristics but unfortunately the high temperatures involved make it expensive, and difficult to prepare.
In order to reduce the temperature, required network-modifying ions are added. Sodium, potassium, and calcium are the most common. The network-modifying ions increase the competition for the oxygen ions, thus loosening the Si-O bonds. Certain other ions may substitute for either the network-forming ions or network-modifying ions; aluminium, zinc, beryllium, lead and ion are a few of these intermediate ions.
3. Constituents of Glass and their Functions:
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The various constituents of glass and their functions are described below:
1. Silica:
It is the principal constituent of glass.
If silica alone is used in the manufacture of glass, it could be fused only at a very high temperature but it would give a good glass on cooling. However, it is imperative to add some alkaline materials (sodium or potassium carbonate) and lime in suitable proportions to make the molten silica glass sufficiently viscous to make it amply workable and resistant against weathering agencies.
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2. Sodium or Potassium Carbonate:
It is an alkaline material and forms an essential component of glass.
It is added in suitable proportion to reduce the melting point of silica and to impart viscosity to the molten glass.
3. Lime:
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It is added in the form of chalk.
It imparts durability to the glass.
In place of lime, sometimes, lead oxide is also added; it makes the glass bright and shining.
4. Manganese Dioxide:
It is added in suitable proportion to correct the colour of glass due to the presence of iron in raw materials of glass.
It is also called ‘Glass maker’ soap.
5. Cullet:
It is the old broken glass of the same type as that which is intended to be prepared.
It is added in small quantity to provide body to the glass.
6. Colouring Substance:
While manufacturing a coloured glass, a suitable colouring substance is added at fusion stage to provide the desired colour to the glass.
The various colouring substances for manufacturing glass of different colours are given below:
4. Properties of Glass:
Following are the properties of glass:
1. No definite crystalline structure.
2. No sharp melting point.
3. Absorbs, refracts or transmits light.
4. Affected by alkalies.
5. An excellent electrical insulator at elevated temperatures.
6. Extremely brittle.
7. Available in beautiful colours,
8. Not affected by air or water.
9. Not easily attacked by ordinary chemical reagents.
10. Capable of being worked in several ways.
11. Can take up a high polish (and may be used as substitute for very costly gems).
12. Possible to weld pieces of glass by fusion.
13. As a result of advancement made in the science of glass production, it is possible to make glass lighter than cork or softer than cotton or stronger than steel.
14. Glass can be cleaned easily by any of the following methods:
(i) Applying methylated spirit.
(ii) Rubbing finely powdered chalk.
(iii) Rubbing damp salt for cleaning paint spots.
(iv) Painting the glass panes with lime-wash and leaving it to dry and then washing with clean water.
Commercial glass must meet the following requirements:
(i) The material must melt at commercially obtainable temperature. Fused silica cools to a glass that is superior to ordinary glass, but the temperature required to melt it is so high that its production is expensive and its use restricted.
(ii) The molten mixture must remain in the amorphous or non-crystalline condition after cooling.
(iii) The fluidity of molten glass must persist to a sufficient extent to permit the formation of desired shapes while the glass is cooling.
(iv) The glass must be reasonably permanent in the use for which it is intended. Glass with a high proportion to sodium oxide is more readily attacked by water and acids than glass that contains less sodium oxide and more lime and magnesia. Glasses low in basic oxides are less readily attacked.
Mechanical Properties:
Ranges of Physical Properties of Glasses:
Refractive index- 1.46 – 2.179
Specific index- 2.215-8.120
Compressive strength- 600 – 1200 MN/m2
Tensile Strength- 27 – 700 MN/m2
Thermal Conductivity- 0.0018 – 0.0028 cal/cm °C/sec.
Expansion coefficient- 8 x 10-7 – 140 x 10-7 cm/cm/°C
Softening point- 500 – 1510°C
Annealing point- 350-890°C
Volume resistivity- 108 – 1018 ohms/cm3
Dielectric constant- 3.7 – 16.5
Strength:
Glass is weak in tension and it always fails in tension no matter how stress is applied. Nevertheless, glass is four to six times as strong in compression as it is in tension and it should, therefore, be used under compressive loads. Strength depends upon the factors such as surface conditions, cross-section and amount of addition etc.
Hardness and Brittleness:
The hardness of glass cannot be measured by the Brinell or Rockwell machines because the test specimen fails when the surface is ruptured. Yet some glasses will scratch steel. The relative value of hardness can be evaluated by scratch or abrasion test. For ordinary purpose the hardness of glass is a function of its tensile strength.
The concept of brittleness has not been clearly defined. When glass is fixed it bends only a little, then shatters. There is no plastic deformation, and the curve of the stress-strain diagram is straight until the test specimen breaks.
Thermal Endurance:
The lower the coefficient of expansion and the thinner the piece the less likely it is to break under thermal shock. Glass does not break from thermal causes but from the tensile stresses set up by the temperature gradient.
Electrical Properties:
1. Conductivity:
Conductivity depends on composition, temperature and surface conditions. Sodium and potassium ions are largely responsible for imparting electrical conduction, but the function of the calcium ions or the mechanism of conduction in an alkali-free glass is not known. Na2O and K2O increase the conductivity. Al2O3, increases the conductivity slightly; ZnO, PbO, MgO, Fe2O, BaO, B2O2 and CaO decrease the conductivity.
The presence of a film on the surface glass affects the surface resistivity to a marked degree. In high alkali glasses the surface conductivity may exceed the volume conductivity. The presence of CO2, SO2, H2S solids and mineral soils also affect the surface conductivity.
In making conductivity tests on glass a precise value is seldom obtained because of the influence of the surrounding medium. Even oil immersion will not give the true value because it has been shown that such a test is a measure of the oil, which fails first, bringing about the failure of the glass. Care in eliminating the edge effects and other conditions show that glass has puncture values several times higher than that obtained in service. The practical limit must take the operating conditions into account.
In the usual method of testing glass for its electrical conductivity the glass Specimen is placed between electrodes. Such a system consists of a capacitor with glass as the dielectric medium. Upon charging such a capacitor, the initial charging current is high and then drops off. The charging current rapidly diminishes and straightens out, approaching a low but finite value.
The discharge current behaves in like manner. If the charged capacitor is short circuited, the initial discharge current is high and drops off rapidly, approaching a low but finite value. The residual charge may be removed by short-circuiting the capacitor. It may be necessary to repeat this process several times. Because of this behaviour, the measurement of electrical resistance may be in error, and the time interval should be stated.
2. Dielectric Constant, Power Factor, Dielectric Loss:
The dielectric constant K of glass varies from 3.7 to 16.5 and remains relatively constant over a wide frequency, the square of the voltage gradient, and to the product of dielectric constant and the power factor. For conditions of high frequency or high voltage or both, the value of K should be kept as low as possible.
Both the dielectric constant and the power factor can be varied by changes in the glass composition by a ratio of more than 200 to 1. Two general glasses are available those with high dielectric constant and low power factor, and those with low dielectric constant and low power factor.
3. Dielectric Strength:
Electrical breakdown is influenced by the characteristics of the dielectric, the thickness, the duration of the stress, temperature and edge effects, the surrounding medium and characteristics of the voltage, whether direct or alternating; thermal failure of dielectric is a result of internal heating, which causes it to become a conductor. Alternating currents causes failure at a lower voltage than direct currents do.
5. Glass Furnaces:
For the manufacture of glass two types of furnaces are used:
1. Pot furnaces.
2. Tank furnaces.
1. Pot Furnaces:
This type of furnace is used to make small amounts of glass such as optical glass that requires careful control of ingredients and of melting and fining operations. The pots are fire clay crucibles ranging up to 0.9 metres in height and up to 1.5 metres in diameter. They are arranged on edges in a circular or oblong furnace where they are exposed to a steadily increasing temperature.
2. Tank Furnaces:
In a continuous tank a constant level of molten glass is maintained by the addition of raw material or “batch” at the proper rates. The furnace is divided into two compartments- the large melting compartment and the smaller working compartment from which the molten glass is withdrawn.
Molten glass flows from the melting to the working compartment through a small opening at the bottom of the dividing wall. The “day tank” is a smaller tank, designed to complete melting of a small charge in one day. It is a particularly useful type of tank in cases where frequent changes in glass composition are required.
Melting:
When the well-mixed raw materials are heated in the glass furnace, those substances with the lower melting points liquify and act as solvents for other substances, at the same time reacting with them and enabling them to react with each other. The system is very complicated one and the exact sequence of events cannot be determined.
The temperature during melting is kept as high as practicable (1400 to 1500°C) in order to reduce the viscosity. Bubbles of gas (carbon dioxide, steam etc.) that are formed during the heating must escape, and their escape will be slower from a more viscous liquid. In order to remove the gases entirely, the glass must be held for some time at the highest possible temperature.
This is called planning or fining. Additions are often made to the batch to lower its viscosity and to generate gas in large volumes at later stages in the melting process, thus sweeping out the small bubbles along with large ones. Salt cake is often used for this purpose.
The under composed portion forms a liquid layer on the surface of the glass and reacts with the scum of siliceous material that tends to collect there. The working temperature, i.e., the temperature at which the glass has the proper viscosity for the shaping operations, is much lower than the highest temperature during melting, the actual working temperature being dependent on the kind of glass and the type of machine used. The range is roughly from 1000 to 1300°C or higher.
6. Fabrication of Glass:
The various processes involved in the fabrication of glass are enumerated and described below:
1. Blowing
2. Flat drawing
3. Rolling
4. Pressing into moulds
5. Casting, and
6. Spinning.
1. Blowing:
In this process, a blow-pipe, 1.5 in to 2 m long and 12 mm diameter is used. One end of the blow-pipe is dipped in a molten mass of glass and a lump (of about 50 N weight) is taken out. The operator then blows vigorously from other end of the blow pipe (it can also be done with the help of a compressor); this blowing causes the molten mass to assume a cylindrical shape. It is then heated for ten seconds and is blown again.
The blowing and heating are continued till the cylinder of required size is formed. It is then kept on an iron plate disconnected from the blow pipe and a cut is made on its top surface longitudinally by a diamond glass cutter so that by gravity the two ends of the cut cylinder will fall out, forming a rectangular thin sheet of glass.
2. Flat Drawing:
In this process, the molten glass in viscous form is drawn in the form of a plate by moving an iron bar side way through it. The plate is then passed over a large rotating roller which helps it in spreading out in a thin sheet.
3. Rolling:
Rolling is a carried out in two ways:
(i) In one method the molten glass is poured on a flat iron casting table and it is then turned flat with the aid of a heavy iron roller.
(ii) In another method, the molten mass of glass is passed between heavy iron rollers and flat glass plate of uniform thickness is obtained.
4. Pressing into Moulds:
In this process, the molten glass is pressed into moulds by machines. Such a glass is stronger and more durable than unpressed glass. This process is used for hollow glass articles, ornamental articles etc.
5. Casting:
In this process, molten glass is poured in moulds and then allowed to cool down slowly. This method is used to prepare large pieces of glass of simple design, mirrors, lenses etc.
6. Spinning:
In this process, the molten glass is drawn into threads or is spun on a high speed wheel operated by power, to produce even finer than cotton threads.
The spun glass has tensile strength equal to that of mild steel. It is very soft and flexible and does not fade, decay or shrink. It is not attacked by acids, fire and vermins.
It is used for producing insulation against heat, sound and electricity.
7. Classification of Glass:
As per composition and properties glass may be classified as:
1. Soda-lime or crown glass
2. Flint glass
3. Pyrex or heat-resistant glass.
1. Soda-Lime or Crown Glass:
It is the cheapest quality of glass.
Its composition, like that of most glass, is not rigidly fixed, but can be varied both as to the amount of ingredients and chemical compounds used.
Composition by Weight:
Sand 75 parts; lime 12.5 parts; soda 12.5 parts; alumina 1 part and waste glass 50 to 100 parts.
It is easily fusible at comparatively low temperatures.
It is available in clean and clear state.
It is possible to blow or to melt articles made from this glass with the help of simple sources of heat.
Uses:
Its principal uses are for window glass, plate glass and container glass (bottles, glass etc.).
2. Flint Glass:
It contains varying proportion of lead oxide to make it suitable for various purposes. Lead provides brilliance and high polish which makes the glass available for special purposes.
Composition by Weight:
Sand 100 parts; red lead 70 parts; potash 32 parts; waste glass 10 parts.
It liquifies at lower temperature than soda-lime glass and has better lustre.
Owing to the ease with which lead compounds are reduced, the glass must be melted in an oxidizing atmosphere.
Uses:
This is potash-lead glass used for better quality of table-wares and for optical glass. It is also used for electric lamps, thermometers, electron tubes, laboratory apparatuses, containers for foods etc.
3. Pyrex or Heat Resistant Glass:
Both soda-lime and flint glasses are unable to withstand sudden temperature changes because of their large coefficients of thermal expansion. The basic oxides that they contain make them susceptible to chemical attack by water and acids.
Elimination of the basic oxides and inclusion of boron oxide produce a glass that is very resistant to thermal shock and to attack by water and acids. The temperature required to melt and fine such a glass is so high that it has to be heated in the electric arc. The familiar Pyrex glasses, which are used extensively for cooking utensils and laboratory wares, are borosilicate glasses.
Composition by Weight:
Silica 80 parts; boron oxide 14 parts; sodium oxide 4 parts; alumina 2 parts, with traces of a potassium oxide, calcium oxide and magnesium oxide.
8. Special Types of Glasses:
1. Annealing Glass:
To prevent glass articles becoming too brittle and falling into pieces at the slightest shock, they are kept while still hot in an annealing furnace to cool very slowly. The longer the annealing period, the better, the quality of the glass.
2. Sheet Glass:
It is roughly composed of 100 parts sand, 35 parts limestones or chalk, 40 parts soda and 50 parts waste glass.
It is made by blowing glass into hollow cylinder, splitting the cylinder and finally flattening it over a plane surface. It is manufactured in thickness varying from 1.5 to 5 mm and sizes upto 1.5 x 1 metre.
Uses:
It is generally used for doors and windows.
3. Plate Glass:
Its composition is- White sand 100 parts; soda carbonate 33 parts; slaked lime 14 parts; manganese peroxide 0.15 part and waste glass 100 parts.
It is made by pouring white hot glass over an iron table and rolling it to a uniform thickness under heavy roller.
It is thicker than sheet glass, and its thickness varies from 5 to 25 mm and sizes upto 275 cm x 90 cm.
It is stronger and more transparent to sheet glass.
In modern glass-fabrication, rolled plate glass is annealed because glass cooled normally is brittle.
Plate glass includes transparent, translucent, opaque and structural glasses.
Uses:
(i) It is used for making looking-glass, wind screens of motors, car skylights and glass houses.
(ii) It is also used for sales counter and table tops after being laminated with plywood or metal sheet.
4. Fluted Glass:
When there are corrugations on one side of the plate glass then it is known as fluted glass. The other side is wavy but smooth. The light is admitted without glare of the sun.
Uses:
It is used in situations where it is desirable to secure privacy without obstruction of light. Horizontal ribs give more light in the middle and less at the sides, while upright ribs give more light at the sides and less in the middle. It is thus more used for skylight roofs and for windows of railway stations and factories.
5. Ground Glass:
It is made either by grinding (usually by sand blasting) one side, or by melting powdered glass upon it.
This glass is mostly translucent.
This type of glass is also known as frosted glass or obscured glass.
Uses:
This glass is used in situations where light is required without transparency. It is normally used for window panes and bath room ventilators etc.
6. Wired Glass:
It is glass with wire netting or similar strengthening material embedded in it during manufacture. This is why this glass is known as reinforced glass.
It resists fire better than ordinary plate glass.
In case the glass is fractured it does not fall into pieces.
Uses:
It is used for skylight and roofs, also for fire-resisting doors and windows.
7. Safety Glass:
This type of glass is produced by sandwiching sheets of celluloid or other transparent plastic between two sheets of glass and sticking the whole combination together by means of colourless and heat resisting glue.
Wired glass falls into the category of safety glass.
8. Bullet-Proof Glass:
This glass is made of several layers of plate glass and alternate layers consist of vinyl- resin plastic. The outer layers of plate glass are made thinner than the inner layers. The special care is to be taken for heating and cooling of layers during manufacture.
The thickness of this type of glass may vary from 15 mm to 75 mm or more.
This glass will not allow bullet to fierce through it.
Uses:
It is extensively used for glazing bank teller, cages, cashier booths, jewellery stores, display cases etc.
9. Insulating Glass:
It is transparent glass unit composed of two or more plates of glass separated by 6 to 13 mm of dehydrated captive air, hermetically sealed inside, is scientifically cleaned and dried.
Insulating glass provides a high resistance to heat-flow. The sealed air makes the coefficients of heat transmission of the glass low and hence keeps the apartment cool in summer and warm in winter.
10. Foam Glass:
It is prepared from powdered glass to which is added the desired quantity of carbon or any gas which makes the mass porous and light in weight.
This glass is water-proof also.
It can be easily cut and worked with common masonry tools.
Uses:
It is used for sound and heat insulation purposes. It is specially recommended for use in air-conditioning of buildings.
11. Glass Blocks:
These consist of two-halves so fused together as to form a hollow inside.
They provide insulation against heat, cold and noise and are easy to clean.
Uses:
They are widely used for constructing wall partitions.
12. Soluble Glass:
It is prepared by melting quartz sand, grinding and thoroughly mixing it with soda ash, sodium sulphase or potassium carbonate. The melting is carried out in glass tank at a temperature between 1300°C to 1400°C and it takes about 7 to 10 hours. The resultant glass mass flows out from the furnace and it cools rapidly and breaks up into pieces, known as silica lumps.
It is soluble in water, under normal conditions.
The soluble glass in the form of silicate lumps is transported in containers and in the form of liquid, it is transported in barrels or glass bottles.
Uses:
It is used for preparing acid-resistant cement.
13. Ultra-Violet Glass:
It transmits ultra-violet rays effectively even though it is not in the direction of the rays of sun.
It is made from the raw mixture with minimum admixtures of iron, titanium and chrome.
Uses:
It is used in the windows of schools, hospitals etc.
14. Structural Glass:
This type of glass is available in the form of glass-crete square blocks, tiles or lenses in thicknesses varying from 5 mm to 30 mm.
These glass products are hollow, light and transparent.
This type of glass can be sawn, placed and drilled like woodwork, inspite of having general properties of glass.
Uses:
Widely used for pavement lights, partitions, lantern lights; also used for roof covering material in industrial buildings, factories etc.
Glass-Fibre or Glass-Wool:
The usual composition of glass-fibres is that of a soda-lime glass but it may be varied for different purposes.
The glass-fibres are made by letting the molten glass drop through tiny orifices and blowing with air or steam to attenuate the fibres.
They have very high tensile strengths, upto about 2750 N/mm2.
Glass-fibre or glass-wool differs from mineral wool in that it is a glass made to a definite formulation with a uniformity not found in mineral wool.
9. Uses of Glass:
Besides other uses, some of the important uses of glass, based on the recent development in the glass industry, are as follows:
1. The fibre glass reinforced with plastics can be used in the construction of furniture, cars, trucks, lampshades, bath room fittings etc.
2. Glass is used to form a rifle barrel which is lighter and stronger than conventional type.
3. Thousands of items in the body of a guided missile are made of glass.
4. Glass is used in the construction of noses of deep-diving vehicles.
5. Optical glass is finding wide application for the development and advancement of sciences of astronomy and bacteriology.
6. The glass linings are applied on equipments likely to be affected by the chemical corrosion such as valves, pipes, pumps etc.
7. Hollow glass blocks can be used for the construction of the walls and ceilings of the modern homes.
8. These days, it is possible to prepare the colour-changing glass; a window with such a glass will be transparent during the day and it will be a source of light at night.